This invention relates to the field of optical recording and, more particularly, to apparatus for controlling the position of an objective lens in an optical read/write head.
In an optical recording system, data is recorded on an optical recording element in the form of spots or pits arranged along elongated data tracks. Typically, the recording element is in the form of a radiation-sensitive disk which rotates at high speed (e.g. 1800 rpm) past an optical read/write head. the latter serves to focus a laser beam onto the radiation-sensitive surface of the disk, and data is either read or written along closely spaced (e.g. 1 or 2 micron spacing) concentric data tracks. By moving the head in a radial direction, data can be accessed at random from any of the recorded tracks.
In conventional read/write heads, the objective lens used to focus the read/write laser beam is typically mounted for movement in orthogonal directions, i.e. in a first direction perpendicular to the lens axis, and in a second direction parallel to the lens axis. A tracking actuator mechanism, responsive to a tracking error signal, serves to continuously adjust the lateral position of the lens mount so that the focused beam follows a desired data track. Similarly, a focusing actuator mechanism, responsive to a focus error signal, serves to move the objective lens toward and away from the recording element so as to continuously maintain the laser spot in sharp focus. To provide a high frequency response, the objective lens is necessarily of exceptionally low mass, and its movable mounting mechanism has a relatively low spring constant.
In order to minimize the settling time of the objective lens after a rapid radial movement of the read/write head, it is known in the art to provide, in the optical head itself, a transducer for continuously monitoring the instantaneous displacement of the lens axis from the mechanical axis of the write/read head. Such displacement is often represented by a damped oscillating signal which returns to zero only after several milliseconds. This settling time can be dramatically reduced by feeding the output of the lens position-sensing transducer back to the input of a tracking control circuit.
In U.S. Pat. No. 4,302,830, the aforementioned lens position-sensing transducer takes the form of a piezoelectric element. Such element is secured to a leaf spring which supports an objective lens for movement in a tracking direction, i.e. perpendicular to the lens axis. As the leaf spring flexes under the influence of a tracking actuator (which moves the lens support in response to a tracking error signal), such flexure is sensed by the piezoelectric element and a signal is produced which is proportional to the amount of flexure (i.e. lens displacement from a nominal position). Such signal is fed back to a tracking control circuit which controls the tracking position of the lens in a motion feedback manner.
While a piezoelectric lens position transducer may offer certain advantages in sensing the lens position, (e.g. it can also function as a driver for correcting the tracking error), it is relatively difficult to implement with a high degree of reliability. It would be desirable, therefore, to provide a simpler and more reliable mechanism for sensing lens position, one which can detect lens displacement without physically contacting the movable lens support.